1. Technical Field
The invention relates to an elongated optical element with conglutinated components and to a method for making the same.
2. Description of the Related Art
It is very important that elongated optical elements, such as multi-fiber buffer tubes or cables, are impervious to water in the longitudinal direction. The individual components of the optical element have frequently no mutually complimentary surfaces if the components are assembled from units, for example, with circular cross-sections. Cavities and gaps then remain between the components through which humidity can propagate. To create a solid water-tight cross-section, the cavities are frequently provided with a filler, such as petroleum jelly or a powder, which can expand in water. Otherwise, water may enter through joints or damaged areas and spread inside the element and adversely affect its mechanical and optical properties, in particular when the water freezes at low temperatures.
It is known from EP 0 261 000 B1 to fill the cavities with cable materials which can swell in water. The materials seal the cavity when moisture enters and thereby prevent water from spreading further along the cable. For easier handling, the materials are advantageously applied to the surface of a water-tight plastic support which due to its own volume and in cooperation with water-tight expansion members, reduces the cross-sections which have to be filled with the swellable materials.
The published application DE 42 28 956 A1 proposes to secure the fibers of an optical wire to a core with a high tensile strength by embedding the fibers in a layer of a hot-melt adhesive surrounding the core. Securing and arranging the fibers in this manner makes it easier to connect the element, for example, to a terminal. The hot-melt adhesive fills the remaining cavities between the fibers and the core and ensures imperviousness to water in the longitudinal direction. In addition, one or more elements are inserted in a metal or plastic protective tubing which is filled with a sealing compound.
It is known that optical cables can also be sealed using hot-melt adhesives a co-polymer of ethylene and vinyl acetate that swell in water. Such a hot-melt adhesive is available, for example, under the name Macromelt Q4402 from the producer Henkel KGaA (Dusseldorf, Germany). Because this material is expensive, it is generally not used to seal optical elements, in particular for large production quantities.